Many electronic devices use portable power sources such as a cell or a battery. As used herein, the term “cell” refers generally to any power source that may store energy and furnish the stored energy as an electrical voltage and the term “battery” refers generally to multiple cells. Furthermore, cells and batteries may be electrostatic or electrochemical in nature.
Electronic devices are increasingly able to execute multiple functions simultaneously, thereby increasing the power demand of the device and accordingly, the importance of the runtime and lifetime of the battery. For example, laptop computers require increasing amounts of power as more functions are simultaneously used such as DVD-ROMs, wireless networking cards, the processor and the screen. Although there are currently ways to increase the battery runtime such as adjusting the brightness of your screen to a lower intensity, these solutions function within the confines of current power source limitations.
Typical modern battery designs are composed of cells connected in series and parallel combinations. For example, a common configuration of lithium-polymer cells is a three in series, two in parallel configuration. Multiple cells within a battery are often used to extend the battery runtime or simply to meet the power demand of the device. The device may therefore draw power from the cells within the battery simultaneously to meet the power draw demand.
A primary goal of battery management is to estimate the remaining battery capacity. The state of charge of each cell is the cell's remaining capacity, expressed as a percentage. For example, a fully charged cell has a state of charge of 100%, and a fully discharged cell has a state of charge of 0%. Estimation of the state of charge may be referred to as the cell management unit function.
One technique for determining the state of charge is to measure the cell's open circuit voltage. After temperature correction, the open circuit voltage of the cell may provide a reasonable estimate of the state of charge for a given cell chemistry. However, this technique is only useful when the cell is not being charged or discharged. Accordingly, the correlation of open circuit voltage to state of charge may improve as the cells are allowed to rest for longer periods of time.
Another technique for measuring the state of charge is the coulomb counting technique. The coulomb counting technique may be used if a rested cell is not available for measurement. The coulomb counting technique estimates the state of charge by accumulating the amount of charge entering or leaving the battery, as measured by a sense resistor in series with the battery. This technique uses a dead-reckoning algorithm where small errors accumulate with time. Consequently, in an environment where a battery is not allowed to rest, the error in the state of charge estimate will be significant.
Additionally, the state of charge may be used for monitoring cells to assist in controlling the charge of each individual cell in a multiple cell battery. One of the objectives in managing the power draw from a battery with cells in series may be to equalize the state of charge of the individual cells. Failure to equalize or balance the state of charge of individual cells with one another in a battery with cells in series may lead to reduced capacity of the battery. For instance, when charging, the cell with the highest state of charge will terminate charging before other cells are fully charged. Similarly when discharging, the cell with the least charge disables the battery even when other cells still have charge. For this reason, the capacity of a battery with imbalanced cells is less than the sum of the capacities of the individual cells. There are many techniques to address balancing of cells, either by balancing during the charging stage or constant rebalancing between cells, but these techniques often rely on either accurate estimates of cell impedance or state of charge. Batteries with cells that are too far out of balance are typically identified and disabled, leading to lower pack lifetime. A number of factors contribute to cell imbalance, such as aging characteristic differences of the cells or uneven temperature distribution within the system.
There are a number of disadvantages imposed by cell balancing. All cells must have the same or substantially similar capacities and the number of cells in a battery has to be a multiple of the number of cells in series. The temperature gradient across the battery must be constrained to prevent different rates of cells aging. Historically, cell imbalance has been managed by using cell selection to construct batteries. Cell selection includes matching cells that have the same or substantially similar capacities for use in a battery.
Accordingly, there is a need in the art for an improved cell configuration and cell management system that accurately measures state of charge without user initiated rest periods, avoids cell balancing requirements and has improved battery lifetime.